Walking puts all of the abdominal muscles to work. Make sure you swing your arms and contract your midsection while you walk, and maintain a brisk pace. Once you get your body accustomed to a daily walk, you'll hate to go a day without it. Walk for at least thirty minutes each time to achieve the aerobic effect, and be sure to drink plenty of water.
There's no way around the fact that, when it comes to burning more fat, you have to work at it. There is no magic exercise, workout, or pill that will do the job for you. The good news is that it doesn't take much activity to push the body into that fat burning mode. Try incorporating some type of activity every day, even if it's just a quick walk, and build on that over time as it becomes more of a routine. Do that and you're on the way to burning more fat.